How do dendrites form

 
In general, dendrites are to be understood as meaning directionally solidified crystallites with a fir tree-like structure. When the melt solidifies, a so-called trunk is formed first. From here, smaller side branches grow into the melt and the interdendritic spaces. The first dendrite plane formed is poorer in alloy than the dendrite arms that emerge in the further course and the solidified melt in the residual solidification fields (interdendritic space). Depending on the level of origin, when defining dendritic crystals, a distinction is made between primary, secondary and tertiary arms. In the course of the further cooling of the melt, these structures expand further and further until they collide and the melt has solidified. In the end, fir tree-like crystals emerged. A distinction is made between directed, oriented and undirected dendrites. The respective form and arrangement in the solidification structure depends on the cooling conditions (conditions of heat transport).

Formation of dendrites:
Cooling conditions and the level of impurities and / or alloying elements influence the formation of dendrites. Impurities, alloy and trace elements can accumulate in the residual melt or before the solidification front and lower the solidification temperature. The heat conduction in the solid and the liquid phase plays an essential role in the formation of dendrites and their growth. Solidification processes proceed with the release of heat (exothermic process). The heat released during solidification can either be dissipated via the melt or via the solidified and growing crystal.
1. The temperature in the crystal is lower than in the melt.
If the heat is dissipated via the crystal, the solidification front remains flat - smooth-walled solidification. No dendrites are formed.
Reason: If a crystal grows out into the residual melt on the solidification front, it forms back again due to the higher ambient temperature (is more or less melted down again).
2. The temperature in the melt is lower than in the crystal.
In those cases in which the heat is dissipated via the melt (with strong undercooling of the melt), long crystals grow from the solidification front and form the dendrites here. The solidification front is uneven - dendritic solidification.
Reason: If a crystal grows beyond the solidification front into the residual melt, the cold melt crystallizes on it and it can continue to grow quickly, often branching in all directions.